How did riches and power lead to church abuses and then to reforms?

Riches and power led to many popes becoming corrupt. Instead of the king having or monarch having all the power the pope was usually the most powerful. The money that the church had made them want more. So, they would sell things such as indulgences to people which was a card that forgave you for your sin without asking god for it. Because of this corruptness and abuse many reforms were made. For example like Luther's 95 thesis which was a list of 95 things about how corrupt and abusive the church is and what they need to change. This led to many people either forcing the church to reform or leaving it permanently .